HTTP Range header

Question

I was reading http   www w3 org Protocols rfc2616 rfc2616-sec14 html sec14 35 and trying to figure out how to continue a file download   For example  suppose a file is of length 100 bytes and I have all the 100 bytes  However  I don t know what the expected file size should be  so I ask for the file and specify a Range header that looks like this   Range  bytes 100-   Is this a valid Range request

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Contrary to Mark Novakowski answer  which for some reason has been upvoted by many  yes  it is a valid and satisfiable request  In fact the standard  as Wrikken pointed out  makes just such an example   In practice  Apache responds to such requests as expected  with a 206 code   and this is exactly what I use to implement progressive download  that is  only get the tail of a long log file which grows in real time with polling

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It s a syntactically valid request  but not a satisfiable request   If you look further in that section you see      If a syntactically valid byte-range-set includes at least one byte- range-spec whose first-byte-pos is less than the current length of the entity-body  or at least one suffix-byte-range-spec with a non- zero suffix-length  then the byte-range-set is satisfiable  Otherwise  the byte-range-set is unsatisfiable  If the byte-range-set is unsatisfiable  the server SHOULD return a response with a status of 416  Requested range not satisfiable   Otherwise  the server SHOULD return a response with a status of 206  Partial Content  containing the satisfiable ranges of the entity-body    So I think in your example  the server should return a 416 since it s not a valid byte range for that file

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As Wrikken suggested  it s a valid request  It s also quite common when the client is requesting media or resuming a download    A client will often test to see if the server handles ranged requests other than just looking for an Accept-Ranges response  Chrome always sends a Range  bytes 0- with its first GET request for a video  so it s something you can t dismiss   Whenever a client includes Range  in its request  even if it s malformed  it s expecting a partial content  206  response  When you seek forward during HTML5 video playback  the browser only requests the starting point  For example   Range  bytes 3744-   So  in order for the client to play video properly  your server must be able to handle these incomplete range requests    You can handle the type of  range  you specified in your question in two ways   First  You could reply with the requested starting point given in the response  then the total length of the file minus one  the requested byte range is zero-indexed   For example   Request   GET  BigBuckBunny 320x180 mp4  Range  bytes 100-   Response   206 Partial Content Content-Type  video mp4 Content-Length  64656927 Accept-Ranges  bytes Content-Range  bytes 100-64656926 64656927   Second  you could reply with the starting point given in the request and an open-ended file length  size   This is for webcasts or other media where the total length is unknown  For example    Request   GET  BigBuckBunny 320x180 mp4 Range  bytes 100-   Response   206 Partial Content Content-Type  video mp4 Content-Length  64656927 Accept-Ranges  bytes Content-Range  bytes 100-64656926     Tips   You must always respond with the content length included with the range  If the range is complete  with start to end  then the content length is simply the difference   Request       Range  bytes 500-1000  Response      Content-Range  bytes 500-1000 123456  Remember that the range is zero-indexed  so Range  bytes 0-999 is actually requesting 1000 bytes  not 999  so respond with something like   Content-Length  1000 Content-Range  bytes 0-999 123456   Or   Content-Length  1000 Content-Range  bytes 0-999     But  avoid the latter method if possible because some media players try to figure out the duration from the file size  If your request is for media content  which is my hunch  then you should include its duration in the response  This is done with the following format   X-Content-Duration  63 23    This must be a floating point  Unlike Content-Length  this value doesn t have to be accurate  It s used to help the player seek around the video  If you are streaming a webcast and only have a general idea of how long it will be  it s better to include your estimated duration rather than ignore it altogether  So  for a two-hour webcast  you could include something like   X-Content-Duration  7200 00    With some media types  such as webm  you must also include the content-type  such as   Content-Type  video webm    All of these are necessary for the media to play properly  especially in HTML5  If you don t give a duration  the player may try to figure out the duration  to allow for seeking  from its file size  but this won t be accurate  This is fine  and necessary for webcasts or live streaming  but not ideal for playback of video files  You can extract the duration using software like FFMPEG and save it in a database or even the filename   X-Content-Duration is being phased out in favor of Content-Duration  so I d include that too  A basic  response to a  0-  request would include at least the following   HTTP 1 1 206 Partial Content Date  Sun  08 May 2013 06 37 54 GMT Server  Apache 2 0 52  Red Hat  Accept-Ranges  bytes Content-Length  3980 Content-Range  bytes 0-3979 3980 Content-Type  video webm X-Content-Duration  2054 53 Content-Duration  2054 53   One more point  Chrome always starts its first video request with the following   Range  bytes 0-   Some servers will send a regular 200 response as a reply  which it accepts  but with limited playback options   but try to send a 206 instead to show than your server handles ranges  RFC 2616 says it s acceptable to ignore range headers

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For folks who are stumbling across Victor Stoddard s answer above in 2019  and become hopeful and doe eyed  note that   a  Support for X-Content-Duration was removed in Firefox 41   https   developer mozilla org en-US docs Mozilla Firefox Releases 41 HTTP  b  I think it was only supported in Firefox for  ogg audio and  ogv video  not for any other types   c  I can t see that it was ever supported at all in Chrome  but that may just be a lack of research on my part   But its presence or absence seems to have no effect one way or another for webm or ogv videos as of today in Chrome 71   d  I can t find anywhere where  Content-Duration  replaced  X-Content-Duration  for anything  I don t think  X-Content-Duration  lived long enough for there to be a successor header name   I think this means that  as of today if you want to serve webm or ogv containers that contain streams that don t know their duration  e g  the output of an ffpeg pipe  to Chrome or FF  and you want them to be scrubbable in an HTML 5 video element  you are probably out of luck   Firefox 64 0 makes a half hearted attempt to make these scrubbable whether or not you serve via range requests  but it gets confused and throws up a spinning wheel until the stream is completely downloaded if you seek a few times more than it thinks is appropriate   Chrome doesn t even try  it just nopes out and won t let you scrub at all until the entire stream is finished playing

[http] HTTP Range header

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